The present invention relates to a method for generating methane and/or methane hydrate from biomass.
The term “biomass” is understood as meaning plant or animal material. Wood, manure, slurry, straw, grass, algae, sewage sludge and abattoir waste may be cited by way of example.
However, the method should also be suitable for other materials having organic content such as: plastic waste, effluents, refuse, used tires, waste paper, waste oils, organic solvents, fossil biomass (peat, coal, mineral oil).
Attention is drawn to the huge, largely unused energetic potential of slurry in a study recently commissioned by the Swiss Federal Energy Agency (BFE, Switzerland) from Wädenswil University, Scheurer, K.; Baier, U.: “Biogene Güter in der Schweiz. Massen und Energieflüsse” (“Biogenic materials in Switzerland. Volumes and energy flows”), University of Wädenswil, commissioned by the BFE, Biomass Program, Final Report, February 2001. In 1998/99, the entire yield of farmyard manure (dung+slurry) amounted to 2,283 million t DS (dry substance), which is equivalent to an energy content of 37 PJ. In 1998, the fermentation of 4'700 t DS farmyard manure yielded around 48 TJ of energy in the form of biogas, which represents only approx. 0.1% of the entire energy potential in farmyard manure. Moreover, during the fermentation relatively large volumes of non-fermentable solid matter are produced. Woody biomass can practically not be fermented.
In the following description the term “hydrothermal” refers to an aqueous system under pressure and increased temperature, typically near to or above the critical point of water (374° C., 221 bar). Near-critical and supercritical water forms an interesting reaction medium for producing chemical reactions. This medium is suitable in particular for the hydrolysis and conversion of biomass into liquid and gaseous products. Since the transition of a liquid system under pressure into the supercritical range does not constitute a true phase transition, no evaporation enthalpy has to be used for the water contained in the biomass, which is in contrast to gas phase processes (e.g. atmospheric, gasification of wet biomass). Consequently hydrothermal processes have the potential for high levels of thermal efficiency.
The most preferred reaction for the conversion of biomass into methane can be described by way of example for wood with the following stoichiometry:CH1.52O0.64(s)+0.3H2O(g)→0.53CH4(g)+0.47CO2(g)  (1)
Under normal conditions (low water partial pressure) the conversion of biomass with water does not, or not completely, proceed according to Equ. (1), but instead byproducts are produced such as e.g. tars or solid carbon (coke). If the reaction conditions are successfully chosen such that reaction (1) takes place completely, a high level of thermal efficiency can be expected since the reaction (1) is slightly exothermal. The theoretically maximum possible efficiency is 95% (referred to the net calorific value Hu of wood). A system analysis performed by the applicant for a commercial process produced an achievable level of efficiency in the range of 70-80% for wood. This has been described in detail in the literature reference “Vogel, F., and F. Hildebrand, Catalytic Hydrothermal Gasification of Woody Biomass at High Feed Concentrations. Chem. Eng. Trans. 2, 2002, 771-777”. This is significantly higher than the efficiency of other methods for converting wood into methane.
To sum up, in terms of the achievable efficiencies, however, the currently known processes for generating methane from biomass fall short of the theoretical expectations, with the result that they cannot be utilized economically at the present time.